The present invention relates generally to the measurement and control of characteristics of a cluster ion beam and, more particularly, to measuring and/or controlling the average charge state, average cluster ion mass, and/or average cluster ion energy of cluster ions in a gas cluster ion beam.
The use of a cluster ion beam for processing surfaces is known (see for example, U.S. Pat. No. 5,814,194, Deguchi et al., incorporated herein by reference) in the art. For purposes of this discussion, gas clusters are nano-sized aggregates of materials that are gaseous under conditions of standard temperature and pressure. Such gas clusters typically are comprised of aggregates of from a few to several thousand molecules loosely bound to form the cluster. The clusters can be ionized by electron bombardment or other means, permitting them to be formed into directed beams of controllable energy. Such ions each typically carry positive charges of q·e (where e is the electronic charge and q is an integer of from one to several representing the charge state of the cluster ion). Non-ionized clusters may also exist within a cluster ion beam. The larger sized cluster ions are often the most useful because of their ability to carry substantial energy per cluster ion, while yet having only modest energy per molecule. The clusters disintegrate on impact, with each individual molecule carrying only a small fraction of the total cluster ion energy. Consequently, the impact effects of large cluster ions are substantial, but are limited to a very shallow surface region. This makes cluster ions effective for a variety of surface modification processes, without the tendency to produce deeper subsurface damage characteristic of conventional monomer ion beam processing.
Means for creation of and acceleration of such gas cluster ion beams (GCIBs) are described in the reference (U.S. Pat. No. 5,814,194) previously cited. Presently available cluster ion sources produce clusters ions having a wide distribution of sizes, N (where N=the number of molecules in each cluster ion—in the case of monatomic gases like argon, an atom of the monatomic gas will be referred to as a molecule and an ionized atom of such a monatomic gas will be referred to as a molecular ion—or simply a monomer ion—throughout this discussion).
Many useful surface processing effects can be achieved by bombarding surfaces with GCIBs. These processing effects include, but are not necessarily limited to, cleaning, smoothing, etching, and film growth. There is need for improved diagnostic measurements to predict and control the gas cluster ion beam physics that affect processing rates of surfaces and hence permit these rates to be optimized. The fundamentals of GCIB-surface interactions are individual cluster ions impacting a surface, moving and ejecting material by direct sputtering and/or through melting and evaporation, creating nano-scale craters of various depths and diameters. GCIB craters can be on the on the scale of tens to hundreds of Angstroms, and depend on cluster ion mass and velocity. The nature of the craters formed (and thus the surface processing characteristics) by a GCIB depends on the mass of the cluster ions and their impact velocity.
Cluster ion velocity may be measured using time-of-flight techniques (as taught in U.S. Patent Application Publications 2002-0036261A1, Dykstra, Jerald P., and 2002-0070361A1, Mack, et al., for example, which are incorporated herein by reference). Also, by a variety magnetic and electrostatic measurement techniques sensitive to the mass/charge state ratio (m/q), the momentum, energy and mass of ionized clusters can be determined, when using ionization conditions such that the ions are predominantly singly charged, thus assuring that q was known to be approximately one. Additionally, it is known that techniques exist for measuring average mass/charge state ratio,
            (              m        q            )        average    ,or can be based on existing techniques (as taught in U.S. Patent Application Publication 2001-0054686A1, Torti. et al., for example, incorporated by reference).
In many cases, it is found that in order to achieve industrially practical throughputs in GCIB processing, GCIB currents on the order of hundreds to thousands of microamps are required. Recent efforts to increase the intensity and ionization of GCIBs are producing additional higher charge state clusters (q>1). When ionization is performed by electron bombardment, ionization is produced by random electron impacts. In order to produce a high ratio of ionized to non-ionized clusters, the electron impact probability must be high and the resulting charge state distribution follows approximately Poisson statistics, with the approximate probability, P(q), of charge state q given by:
                                                        P              _                        ⁡                          (              q              )                                =                                                    q                q                                            q                !                                      ⁢                          ⅇ                              -                                  q                  _                                                                    ,                            (                  Eqn          .                                          ⁢          1                )            
where {overscore (q)} is the average ionized cluster charge state after leaving the ionizer. Thus an ionized cluster beam with a highly ionized fraction will also be a multiply charged beam. For example, theoretically the average cluster charge state of a GCIB beam where 95% of the clusters are ionized would be 3, with more than 8% of the beam in charge states 6 and higher. However, such highly charged clusters can fragment, resulting in a different charge state distribution. The interaction of the cluster ions with residual gas in the vacuum system can also cause charge exchange reactions and cluster ion fragmentation and so in a practical beam, the precise charge state is not readily predicted. The existence of high charge state clusters cannot be determined using present instrumentation such as magnetic spectrometers, electrostatic spectrometers, RF quadrupole mass spectrometers, time-of-flight, retarding potential mass spectrometers, and pressure gauge measurements, all of which measure either m/q or the energy/charge state ratio, E/q. It should be noted that for ionized cluster m/q on the order of 10000 AMU or less, it is possible to resolve different charge state families in the m/q spectra but this is not practical for more massive cluster ions where m/q states cannot be practically resolved or overlap. Other methods have been used to determine these parameters for very-large very-highly-charged (q>1000) molecules produced by electro-spray techniques, and also for the case of highly-charged atoms in the solar wind or electrostatically accelerated dust particles. These techniques are not applicable to GCIBs because, for the former case, the charge states are too low, and with respect to the latter technique, because the cluster ion's collision energy is nearly all deposited thermally and hence cannot be detected using practical known methods. Thus, improved methods and apparatus are needed to measure and control m, q and E of these cluster ions.
It is therefore an object of this invention to provide methods and apparatus for measuring the average charge state {overscore (q)} of a cluster ion beam.
It is a further object of this invention to provide methods and apparatus for measuring average mass {overscore (m)} and/or average energy Ē of energetic ionized clusters in a beam.
Another object of this invention is to provide an improved method and apparatus for measuring and controlling the properties of a gas cluster ion beam in a GCIB processing system for improved GCIB processing of a workpiece.